1. Field of the Invention
The present invention relates to aluminum hydroxide, and a tire tread rubber composition and a pneumatic tire employing the aluminum hydroxide. More particularly, the present invention relates to aluminum hydroxide having specific characteristics; a tire tread rubber composition having the aluminum hydroxide blended therein, which allows grip performance on a wet road surface (wet grip performance) and abrasion resistance to increase and allows rolling resistance to decrease to improve fuel efficiency; and a pneumatic tire employing the tire tread rubber composition.
2. Description of the Background Art
In recent years, in response to the demands for fuel-efficient automobiles, fuel-efficient tires with decreased rolling resistance have been developed. As a technique to decrease the rolling resistance, carbon black conventionally used as a reinforcing agent for tread rubber has been partially replaced with silica, in an effort to balance the antinomic properties of fuel efficiency and wet grip performance.
When compared to the conventional rubber composition with carbon black blended therein, however, the rubber composition with silica blended therein exhibits various problems in terms of processibility. Specifically, it easily decomposes because of high viscosity of unvulcanizate, and is poor in dimensional stability after extrusion. Thus, a tread rubber composition satisfying both the processibility and the performance has been desired.
Several techniques for improving the wet grip performance have also been proposed. One of such techniques is to increase a glass transition temperature (Tg) of a rubber component, or, to increase loss tangent (tanxcex4 value) at 0xc2x0 C. Another technique is to blend carbon black of small particle size into a rubber composition with high loading. If the glass transition temperature (Tg) is increased, however, low temperature performance deteriorates and the rolling resistance increases. The rubber composition heavy-loaded with the carbon black of small particle size also suffers a disadvantage that its rolling resistance increases.
An object of the present invention is to provide a filler for tire, and a tread rubber composition and a pneumatic tire employing the same filler, that can solve the above-described problems by allowing wet grip performance and abrasion resistance to increase, rolling resistance to decrease for improvement of fuel efficiency, and achieving superior processibility at the same time.
According to an aspect of the present invention, aluminum hydroxide is provided which has a loosed bulk density of not more than 0.60 g/cm3, a DOP oil absorption of at least 70 cm3/100 g and less than 250 cm3/100 g, and a BET specific surface area of at least 30 m2/g and not more than 350 m2/g. Particularly, the loosed bulk density of at least 0.10 g/cm3 and not more than 0.35 g/cm3 is preferable.
The aluminum hydroxide preferably has a crystal structure of boehmite type, having a crystal size of boehmite (002) plane of at least 5 nm and not more than 20 nm.
According to another aspect of the present invention, a tire tread rubber composition is provided which is obtained by blending and kneading 5-150 parts by weight of the aluminum hydroxide as described above with 100 parts by weight of a rubber component.
The rubber component is preferably composed of at least 20 parts by weight of styrene-butadiene rubber having a glass transition temperature (Tg) of not more than xe2x88x9227xc2x0 C. and at least 20 parts by weight of diene type rubber including at least one kind of rubber selected from natural rubber, polyisoprene rubber and polybutadiene rubber. In the tire tread rubber composition, 5-60 parts by weight of carbon black having a BET specific surface area of at least 60 m2/g is preferably blended with respect to 100 parts by weight of the rubber component. Further, in the tire tread rubber composition, 2-20% by weight of silane coupling agent is preferably blended with respect to the weight of the aluminum hydroxide.
Alternatively, the tire tread rubber composition may employ a rubber component that includes at least 60 parts by weight of styrene-butadiene rubber having a styrene content of 20-60% by weight. 10-100 parts by weight of carbon black having a BET specific surface area of at least 60 m2/g may be added with respect to 100 parts by weight of this rubber component, and/or 2-20% by weight of silane coupling agent may be blended with respect to the weight of the aluminum hydroxide.
According to a further aspect of the present invention, a pneumatic tire is provided which employs the tire tread rubber composition as described above.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.
The aluminum hydroxide according to the present invention has a loosed bulk density, as measured conforming to JIS H1902, of not more than 0.60 g/cm3, and preferably at least 0.10 g/cm3 and not more than 0.35 g/cm3. If it is greater than 0.60 g/cm3, abrasion resistance of the rubber composition considerably deteriorates. If it is too low, torque when kneading the aluminum hydroxide and the rubber increases, thereby degrading the workability.
Further, the aluminum hydroxide described above has a DOP oil absorption, as measure conforming to JIS K6221, of at least 70 cm3/100 g and less than 250 cm3/100 g, and preferably at least 90 cm3/100 g and not more than 150 cm3/100 g. If it is out of such a range, the rubber composition suffers degradation of its abrasion resistance.
Still further, for the purposes of achieving a rubber composition having an effect to decrease the rolling resistance and exhibiting sufficient grip performance, the aluminum hydroxide has a BET specific surface area, as measured by nitrogen adsorption, of at least 30 m2/g and not more than 350 m2/g, preferably at least 30 m2/g and not more than 200 m2/g, and more preferably more than 100 m2/g and not more than 200 m2/g. If it exceeds 350 m2/g, torque when kneading the aluminum hydroxide and the rubber may increase, thereby degrading the workability.
Moreover, for the purposes of further improving the grip performance, the abrasion resistance and the effect to decrease the rolling resistance of the rubber composition, the aluminum hydroxide of the present invention preferably has a crystal structure of boehmite type. More preferably, the crystal size of boehmite (020) plane is at least 5 nm and not more than 20 nm.
The crystal size was calculated as follows. Peaks of the boehmite (020) planes were measured from the profile obtained using an X-ray diffractometer. For these peaks of the crystal planes, fitting was conducted, based on the Gaussian distribution, using software for xe2x80x9cmulti-peak separationxe2x80x9d of RINT 2100. Using the half-value width of the calculated result and the peak angle obtained by the barycentric method, the crystal size was calculated by the Scherrer""s formula. The measurement conditions for the X-ray diffraction were as follows.
Measurement device: Rint-2100 V from Rigaku International Corporation.
Measurement conditions: Cu target; Voltagexc3x97Current=40 kVxc3x9740 mA; Slit: DS1xc2x0xe2x88x92SS1xc2x0xe2x88x92RS 0.3 mm; Scan mode: continuous; Scan speed=2xc2x0/min; Scan step=0.010xc2x0/step; Scan axis: 2 xcex8/xcex8; Scan range: 2-70xc2x0; and Rotation speed: 0 rpm.
A blended amount of the aluminum hydroxide described above is 5-150 parts by weight, preferably 5-80 parts by weight, and particularly 5-60 parts by weight, with respect to 100 parts by weight of the rubber component described above. If it is less than 5 parts by weight, the decrease of the rolling resistance by virtue of such addition is not adequate, and the grip performance against a wet road surface is improved only to a small extent. If the blended amount exceeds 150 parts by weight, viscosity of the rubber composition becomes too high, which deteriorates the processibility as well as the abrasion resistance.
One way of producing the aluminum hydroxide of the present invention is as follows. Aluminum alkoxide is hydrolized to obtain slurry of aluminum hydroxide, which is passed through a continuous wet grinder or the like to obtain suspension. The obtained suspension is alkalinized, then subjected to heat treatment at about 100xc2x0 C. to about 140xc2x0 C. for about 10 hours to about 100 hours, and then dried using a flash dryer or the like. In this method of producing the aluminum hydroxide, it is preferable that the suspension having undergone the heat treatment is subjected to solid-liquid separation to extract the solid content (aluminum hydroxide), and then the solid content is washed to remove impurities.
In the present invention, various kinds of rubber that are generally used for a tire tread rubber, e.g., natural rubber, polyisoprene rubber and polybutadiene rubber, may be used as a rubber component.
The rubber component employed in the present invention preferably includes at least 20 parts by weight of styrene-butadiene rubber having a glass transition temperature (Tg) of not more than xe2x88x9227xc2x0 C. Conventionally, in order to balance the antinomic properties of improved wet grip performance and decreased rolling resistance, styrene-butadiene rubber with a relatively high glass transition temperature (Tg) has been employed. In this case, however, there is a limit for the improvement of the rolling resistance, and the abrasion resistance tends to be degraded. Thus, according to the present invention, the glass transition temperature (Tg) of styrene-butadiene rubber is made not to exceed xe2x88x9227xc2x0 C. so as to improve the rolling resistance. For the purposes of maintaining good wet grip performance, the styrene-butadiene rubber is preferably made to have a glass transition temperature in a range between xe2x88x9230xc2x0 C. and xe2x88x9250xc2x0 C., and more preferably, at least 50 parts by weight thereof is blended in the rubber component.
When employing such a rubber component, the aluminum hydroxide is preferably added 5-60 parts by weight with respect to 100 parts by weight of the rubber component.
In this case, at least 20 parts by weight of at least one kind of rubber selected from natural rubber (NR), polyisoprene rubber (IR), low cis-polybutadiene rubber (low cis-BR) and high cis-polybutadiene rubber (high cis-BR) is also included as another rubber component. Preferably, these rubber components each have a glass transition temperature (Tg) of not more than xe2x88x9227xc2x0 C., and work together with the above-described styrene-butadiene rubber to improve the wet grip performance, rolling resistance and abrasion resistance, totally in a well balanced manner.
As another embodiment of the present invention, the rubber component being used for the tire tread rubber composition preferably includes styrene-butadiene rubber having a styrene content of 20-60% by weight. If the styrene content is less than 20% by weight, the grip performance in the low and high temperature regions is not improved. If it exceeds 60% by weight, block rigidity increases, so that the xe2x80x9cbitexe2x80x9d of the rubber on contact with the road surface becomes poor, making it impossible to achieve a desired gripping force. In particular, the styrene content of 30-45% by weight is preferable. Such styrene-butadiene rubber is synthesized by emulsion polymerization, solution polymerization or the like.
When employing such a rubber component, the aluminum hydroxide is preferably blended 5-80 parts by weight with respect to 100 parts by weight of the rubber component.
Here, other rubber components that may be used for the tire tread rubber composition include: natural rubber (NR), high cis 1,4 polybutadiene rubber, low cis 1,4 polybutadiene rubber, styrene-butadiene rubber (SBR) other than as described above, polyisoprene rubber (IR), butyl rubber (IIR), halogenated butyl rubber, acrylonitrile-butadiene rubber (NBR), acrylonitrile-styrene-butadiene rubber, chloroprene rubber, ethylene-propylene copolymer rubber, styrene-isoprene copolymer rubber, styrene-isoprene-butadiene copolymer rubber, isoprene-butadiene copolymer rubber, chlorosulfonic polyethylene, acrylic rubber, epichlorohydrin rubber, silicone rubber and urethane rubber. Any of them may be used alone, or at least two kinds thereof may be blended together in any arbitrary proportions. In particular, for the purposes of improving processibility and abrasion resistance, NR, BR, SBR, IR, styrene-isoprene-butadiene copolymer rubber and the like are preferable.
Preferably, at least 60 parts by weight of the styrene-butadiene rubber having a styrene content within the above-described range is blended into the rubber component for use in the present invention. If the blended amount of the styrene-butadiene rubber is less than 60 parts by weight, the grip performance in the low and high temperature regions cannot be improved.
Further, the carbon black being blended into the tread rubber composition of the present invention preferably has a BET specific surface area, as measured by nitrogen adsorption, of at least 60 m2/g, preferably 70-220 m2/g, and more preferably 70-200 m2/g. The BET specific surface area of less than 60 m2/g is not preferable, since an adequate level of abrasion resistance cannot be expected. Here, the BET specific surface area was measured conforming to the ASTMD 4820-99 method.
If the carbon black is being blended into a rubber component including at least 60 parts by weight of styrene-butadiene rubber having a styrene content of 20-60% by weight, the blended amount of the carbon black is 10-100 parts by weight, preferably 30-100 parts by weight, and more preferably 40-100 parts by weight, with respect to 100 parts by weight of the rubber component. If the weight of the carbon black is less than 10 parts by weight, abrasion resistance is deteriorated. If it exceeds 100 parts by weight, viscosity of the rubber increases, thereby degrading the processibility.
In the case where the carbon black is being blended into a rubber component including at least 20 parts by weight of styrene-butadiene rubber having a glass transition temperature (Tg) of not more than xe2x88x9227xc2x0 C., the blended amount of the carbon black is 5-60 parts by weight, preferably 10-60 parts by weight, and more preferably 20-60 parts by weight, with respect to 100 parts by weight of the rubber component. If it is less than 5 parts by weight, abrasion resistance becomes poor. If it exceeds 60 parts by weight, viscosity of the rubber increases, thereby deteriorating the processibility.
Still further, for the purposes of further reinforcing the aluminum hydroxide, 2-20% by weight of silane coupling agent may be added with respect to the blended amount of the aluminum hydroxide. The silane coupling agents that may be used here include: bis(3-triethoxysilylpropyl)tetrasulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(3-trimethoxysilylpropyl)tetrasulfide, bis(2-trimethoxysilylethyl)tetrasulfide, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2-mercaptoethyltrimethoxysilane, 2-mercaptoethyltriethoxysilane, 3-nitropropyltrimethoxysilane, 3-nitropropyltriethoxysilane, 3-chloropropyltrimethoxysilane, 3-chloropropyltriethoxysilane, 2-chloroethyltrimethoxysilane, and 2-chloroethyltriethoxysilane. Among them, bis(3-triethoxysilylpropyl)tetrasulfide is preferable for balancing the cost and the effect expected by adding the coupling agent.
The rubber composition of the present invention may be used together with other fillers, such as silica, clay and the like. It may also be blended, if necessary, with various kinds of additives including process oil, antioxidant, stealic acid, zinc oxide and wax, and of course with vulcanizing agents such as sulfur, vulcanization accelerator and the like.